Switching converters with lower output voltage and higher output current are required in power solutions of high performance CPU. Multi-phase switching converters are widely used in these applications because of its good performance.
FIG. 1 is a block diagram of a prior multi-phase switching converter 100. The multi-phase switching converter 100 comprises N switching circuits, wherein N is an integer larger than or equal to 2. The input terminals of the N switching circuits are configured to receive an input voltage VIN, the output terminals of the N switching circuits are coupled together to provide an output voltage VOUT. The comparator CMP1 compares the output voltage VOUT with a reference signal VREF to generate a comparison signal CMPO. The control circuit generates control signals CTRL1˜CTRLN based on the comparison signal CMPO to control the ON and OFF switching of the N switching circuits. Generally, when the comparison signal CMPO is logical high which indicates the output voltage VOUT is smaller than the reference signal, the control circuit turns on the corresponding switching circuit to provide energy to the load. The switching circuit is turned off by the control circuit when its on-time ton reaches a predetermined time threshold.
In prior art, the control circuit can turn on the following switching circuit based on the comparison signal CMPO only after the off-time of the current switching circuit reaches a minimum off-time tmin—off—time, so the N switching circuits can be turned on successively. However, the duty cycle of the multi-phase switching converter is limited. The multi-phase switching converter can not be used in the applications with duty cycle larger than (100/N) %. The maximum duty cycle Dmax can be described as:
                              D          max                =                                            t              on                                      N              ⁡                              (                                                      t                    on                                    +                                      t                                          min_off                      ⁢                      _time                                                                      )                                              <                                    (                              100                N                            )                        ⁢            %                                              (        1        )            
FIG. 2 is a working waveform of a prior 4-phase switching converter. According to the formula (1), the maximum duty cycle Dmax of the 4-phase switching converter is smaller than 25% and the 4-phase switching converter can not be used in the applications with duty cycle larger than 25%.